1. Field of the Invention
The present invention relates to a Ni-base alloy weld metal, and a strip electrode and a welding method that can be used to produce the weld metal. More particularly, the present invention relates to a Ni-base alloy weld metal suitable for corrosion-resistant structures, such as reactor pressure vessels and chemical reactors, a strip electrode, and a welding method.
2. Description of the Related Art
In structures, such as reactor pressure vessels and chemical reactors, submerged arc welding or electroslag welding by means of a strip electrode overlay welding method is applied to a portion requiring high corrosion resistance, such as an inner surface. For high-temperature corrosion-resistant equipment, such as pressurized water reactor nuclear power plants, although Ni-15Cr alloys having excellent stress corrosion cracking resistance in high-temperature and high-pressure water have been used as weld metals, Ni-30Cr alloys having further excellent stress corrosion cracking resistance are being increasingly used.
Various studies on welding for forming a Ni-30Cr alloy using Ni-30Cr alloy filler metal have been performed in order to improve welding performance and hot cracking resistance (Japanese Unexamined Patent Application Publication No. 8-174270, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2003-501557 (U.S. Pat. No. 6,242,113), and Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2008-528806 (U.S. Patent Application Publication No. 2008/0121629)). For example, in the case of a covered electrode for a Ni-base Cr-rich alloy described in Japanese Unexamined Patent Application Publication No. 8-174270, N is added together with W and V, and the amounts of elements that affect weld cracking, such as Ti, Al, P, S, Si, and O, are specified.
In the case of a Ni—Cr—Fe alloy filler metal described in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2003-501557 (U.S. Pat. No. 6,242,113), the amounts of low-melting-point elements, such as S and P, are specified in order to suppress solidification cracking. Japanese Unexamined Patent Application Publications (Translations of PCT Applications) No. 2003-501557 (U.S. Pat. No. 6,242,113) and No. 2008-528806 (U.S. Patent Application Publication No. 2008/0121629) propose a method for suppressing cold cracking that involves the addition of particular amounts of Nb, Zr, and B so as to improve high temperature strength and ductility.
The inside of a multi-pass welded portion in overlay welding or joint welding using a Ni-30Cr alloy filler metal tends to have microcracks. This intergranular fracture is referred to as a “ductility-dip reheat crack”, as distinguished from a “solidification crack” during the solidification of weld metal. Ductility-dip reheat cracks characteristically occur when a solidified weld is reheated to a temperature less than or equal to its melting point in the subsequent pass. When a Cr-rich Ni-base alloy weld metal containing approximately 30% by mass or more Cr is repeatedly reheated during welding, coarse Cr carbide precipitates at grain boundaries and reduces grain boundary strength, that is, the bonding strength between adjacent crystal grains. Upon the application of a tensile or shear thermal stress during welding, the grain boundaries are cleaved to have ductility-dip reheat cracks.
Precipitation of Cr carbide at grain boundaries increases the susceptibility of the Ni-base alloy weld metal to intergranular corrosion. Cr carbide principally precipitates at grain boundaries at a temperature in the range of 430° C. to 900° C. In particular, a high heat input and a low cooling rate, as in electroslag welding or submerged arc welding using a strip electrode, result in very high susceptibility to Cr carbide and low corrosion resistance. Furthermore, use of a weld metal having a lower tensile strength than a base material limits the designs of equipment and structures. Thus, a weld metal should have substantially the same tensile strength as the base material.
Existing Ni-base alloy filler metals have unsatisfactory performance in high-heat-input submerged arc welding or electroslag welding by means of a strip electrode overlay welding method. For example, use of an arc welding electrode having a high N content described in Japanese Unexamined Patent Application Publication No. 8-174270 results in the precipitation of a large amount of nitride in the weld metal at high temperature, thereby making the weld metal brittle.
A Ni—Cr—Fe alloy described in Japanese Unexamined Patent Application Publications (Translations of PCT Applications) No. 2003-501557 (U.S. Pat. No. 6,242,113) and No. 2008-528806 (U.S. Patent Application Publication No. 2008/0121629) has insufficient solidification cracking resistance and ductility-dip cracking resistance. In particular, these patent documents did not study on solidification cracking resistance in high-heat-input submerged arc welding or electroslag welding having a welding heat input of 40 kJ/cm or more by means of a strip electrode overlay welding method. Thus, there is a demand for a strip electrode that can produce a weld metal having excellent hot cracking resistance, tensile strength, and corrosion resistance for use in high-heat-input submerged arc welding or electroslag welding.